Modulator system

ABSTRACT

A floating deck modulator circuit for the modulation of an electron tube and particularly adapted for x-ray tubes employing a control grid. The modulator circuit utilizes a pair of high voltage electron tubes serially connected through a biasing network for providing a bias voltage between the grid and cathode of the tube. A single pulsing, or keying, circuit is directly connected to one of the electron tubes and coupled to the other electron tube by a transformer which insulates the circuit from high voltage. The modulator circuitry is adapted to be enclosed within an x-ray tube housing, and coupling of the modulation circuitry through the housing to a remote control unit is provided by a modulated light beam which accomplishes coupling across a high difference of potential.

"United States Patent 1191 MacLachlan et al.

[ Apr. 30, 1974 MODULATOR SYSTEM [75] Inventors: Robert K. MacLachlan, Stratford;

Derek Chambers, Framingham, both of Mass.

[73] Assignee: Machlett Laboratories,

Incorporated, Springdale, Conn.

[22] Filed: May 5 1972 [21] Appl. No.: 250,829

Related US. Application Data [62] Division of Ser. No. 38,276, May 18, 1970.

[52] US. Cl 332/13, 332/7, 328/64, 328/232 [51] Int. Cl H03c 3/30 [58] Field of Search 332/7, 13, 25, 58; 328/64, 328/232 [56] References Cited UNITED STATES PATENTS 3,339,146 8/1967 Gorski et a1. 328/232 3,603,887 9/1971 Doss 332/13 X 3,150,331 9/1964 Rambo ..332/7 SIGNAL SOURCE 10/1963 Stanley 332/7 X 7/1963 Dodington 332/7 Primary Examiner-Alfred L. Brody Attorney, Agent, or FirmJoseph D. Pannone; Harold A. Murphy; Milton D. Bartlett [57] ABSTRACT A floating deck modulator circuit for the modulation of an electron tube and particularly adapted for x-ray tubes employing a control grid. The modulator circuit utilizes a pair of high voltage electron tubes serially connected through a biasing network for providing a bias voltage between the grid and cathode of the tube. A single pulsing, or keying, circuit is directly connected to one of the electron tubes and coupled to the other electron tube by a transformer which insulates the circuit from high voltage. The modulator circuitry is adapted to be enclosed within an x-ray tube housing, and coupling of the modulation circuitry through the housing to a remote control unit is provided by a modulated light beam which accomplishes coupling across a high difference of potential.

3 Claims, 4 Drawing Figures .1 [HIGH VOLTAGE "l I GENERATOEG 44 42 l'1 I 4a 1 mmnmrxso 1974 3.808.559

sum 1 0r 3 ggi 'm MODULATOR V LOAD 32 30 F/G 1 20 FAULT HIGH 1 DETECTOR VQLTAGE INTE LOCK GENERATOR FAULT DETECTOR INTE I'QLOCK 15 SIGNAL SOURCE I GENERATOR :sJaosLsss Pmmimraao m4 sum 3 or 3 BACKGROUND OF THE INVENTION This invention relates to circuitry for modulating electron tubes, and more particularly to a relatively low voltage pulsing circuit in combination with a pair of high voltage electron tubes for supplying a preselected voltage to a modulating electrode of the electron tube.

Electron tubes such as x-ray tubes utilize high voltages, on the order of 150 kilovolts, between anode and cathode. Grid bias for electron tubes range from a few kilovolts in the case of some x-ray tubes to tens of kilovolts in some traveling wave tubes, and, accordingly, modulators which apply grid to cathode voltages for inducing states of conduction and noncondu'ction within such tubes must necessarily be able to supply a voltage within the kilovolt range. While the present invention relates generally to electron tubes, its advantages are most readily apparent in the case of x-ray tubes.

X-ray tubes are frequently operated in an environment such as a hospital, where protection from high voltage is essential both during operation of these tubes as well as during servicing of the tubes. These tubes are frequently operated under pulsed conditions, as in x-ray photography where the instantaneous magnitude of the beam current should be accurately controlled to provide a pulse waveform having a flat top as well as a desired duration; rise time, and fall time to preserve the fidelity of x-ray image. Control of the pulse duration is. provided preferably by relatively low voltage electronic equipment which is accessible to a human operator for selection of the desired pulse duration.

In the past the' control, or modulation, circuitry for such electron tubes has been located at a distance from the tube itself. For example, in a hospital environment where x-ray photography of human beings is performed, an x-ray tube positioned within a lead shielded housing is swingably positioned on an articulated hanger which permits the x-ray tube to be oriented and positioned to direct x-rays in any desired direction. The modulation circuitry has generally been placed at a distance from such electron tubes since anyattempt to place such circuitry within the housing itself would have resulted in an excessive increase in the physical size of the housing with an attendant decrease in the ease of mobility of the housing.

A problem arises in that a relatively long connecting cable with attendant high electrical capacitance has been usedfor interconnecting the modulation circuitry with the x-ray tube. The high capacitance presents a load to the modulation circuitry which must be charged and discharged in establishing the high voltage pulses required for modulating the x-ray tube. Furthermore, this capacitance in combination with electrical transformers commonly employed in modulation circuits may cause a ringing or other undesired modulation of the high voltage pulse thereby degrading the pulse waveform.

Another problem presented by modulation circuitry has been a lack of safety and convenience during repair and maintenance of this circuitry due to the presence of high voltages at a number of locations in the electrical circuit which necessitate special precautions during repair and maintenance.

It is also noted that there are two common methods which have been utilized for modulating the beam of an x-ray tube, namely, by switching the high voltage applied between cathode and anode, or by applying a voltage pulse to the grid. The former method has the disadvantage of increased x-radiation during the pulsing of the x-ray tube since an appreciable time may be required for the high voltage to build up and to discharge during which times the patient is exposed to soft x-rays. The latter method of grid control modulation permits very short pulses of x-radiation for photographing a moving object such as a pulsating heart. It is therefore desirable to provide a modulation circuit adapted for grid modulation.

Accordingly, it is an object of the present invention to provide a modulator of reduced size which produces pulses having an improved rise time and fall time.

It is an object of the present invention to provide a modulation circuit employing direct coupling to permit pulses of both short and long duration.

It is also an object of the present invention to provide for a low capacitance interconnection between the modulation circuitry and the x-ray tube.

It is also an object of the present invention to provide a modulation system in which a control unit is located at a distance from the modulator and is insulated from high voltage.

It is furthermore an object of the present invention to provide a modulation circuit operable without the presence ofa large difference of potential, such as the grid to cathodebias of an x-ray tube, so that repairs and maintenance can be made in a relatively safe manner.

It is another object of the present invention to provide a fail-safe signal in response to bias voltage of an electron tube for interlocking a high voltage supply energizing the electron tube.

SUMMARY OF THE INVENTION The foregoing objects and other advantages are accomplished by a modulation circuit in accordance with the present invention wherein a pair of electronic switches such as electron tubes are serially connected through a biasing network for providing a bias voltage pulse between the input tenninals of a load such as the grid and cathode of an x-ray tube. A single pulsing, or keying,circuit is directly connected to one of the electron tubes and is coupled to the other electron tube by a transformer. The use of the transformer reduces the number of electrical components by eliminating the need for an additional pulsing circuit for the second electron tube, and furthermore, accomplishes a synchronization of the pulsing of the two electron tubes. The transformer also insulates the circuit from high voltage. Due to the absence of high voltage, the components of the keying circuit are of relatively small physical size so that the modulation circuit can be contained within the housing of an x-ray tube. The biasing network is adapted to permit the use of a sensing circuit responsive to the bias voltage. A pair of light sources and light receivers (such as infra-red solid state devices) communicate via a pair of light conduits through the housing to insure operator safety during control of the bias voltage pulse as well as to conduct a signal from the sensing circuit for operation of a high voltage interlock.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned objects and other features of the invention are explained in the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is a block diagram of a modulation system incorporating modulation circuitry in accordance with the invention;

FIG. 2 is a schematic diagram of the modulator of FIG. 1;

FIG. 3 is a detailed block diagram of the modulation system of FIG. 1 showing a fault detector and interlock; and

FIG. 4 shows an x-ray tube housing partially cut away to display the manner of positioning the modulation circuitry of the invention within the housing.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there is shown a modulation system wherein a modulator 22 modulates a load 24 in response to a signal received from signal source 26. The signal provided by signal source 26 is preferably in the form of a pulse 28 having a leading edge and trailing edge. Thus, for example, where load 24 is an electron beam tube and the modulator 22 is adapted to initiate and terminate a pulse of electrons in the beam of such an electron beam tube, the leading edge of pulse 28 conveniently indicates the instant when the pulsing of the electron beam is to be initiated and the trailing edge of pulse 28 indicates the termination of the pulse of the electrons. Where load 24 is for example a travelin g wave tube or an x-ray tube of the form having a grid for focusing and modulating the electron beam, the modulator 22 operates by providing a voltage between the grid and cathode of the load 24. High voltage generator 30 supplies high voltage to load 24 and for example, in the case of a traveling wave tube or an x-ray tube this high voltage is applied between the cathode and the anode. In addition, a fault detection and interlock 32 responsive to voltages and currents appearing in the modulator 22 as will be described hereinafter is utilized to disable the high voltage generator 30 when a fault is detected in the input control circuit to the load; for example, in the case where load 24 is an x-ray tube such a fault might be an improper grid voltage bias condition or possibly an arcing within the tube.

Referring now to FIG. 2 there is shown a schematic diagram of the modulator 22 interconnected with the load 24, the high voltage generator 30, the signal source 26 and the fault detector and interlock 32. The load 24' is shown, by way of example, as an x-ray tube 34 having an anode 36, a cathode 38 and a grid 40. Power for the x-ray tube 34 is provided by the high voltage generator 30 utilizing typically a high voltage transformer having an output winding which is center tapped to ground, the output circuit being represented herein by a pair of batteries 42 and 44 connected together at point 46 and there grounded to earth ground 48. The positive terminal of battery 44 is connected to the anode 36 of x-ray tube 34. The negative terminal of battery 42 is connected to the junction of two sources of electric power represented by batteries 50 and 52 which serve as bias supplies for x-ray tube 34. The voltages provided by these batteries are selected in accordance with the load 24. Batteries 42 and 44 provide preferably equal voltages each of value kilovolts, battery 50 provides a voltage of typically 800 volts and battery 52 provides a voltage of approximately 6 kilovolts. In addition two other sources of electric power represented by batteries 54 and 56 are utilized, battery 54 providing a voltage of typically 28 volts and battery 56 providing a voltage of typically 1 30 volts as is required by the modulator circuit elements. Battery 52 connects with the common junction of battery 54 and battery 56 so that the modulator circuitry is essentially floating at a negative potential of approximately 6 kilovolts relative to the cathode 38 of x-ray tube 34. In practice the electric power of battery 52 is provided by an external generator which also supplies filament heater voltage, not shown in FIG. 2, for x-ray tube 34. The source of voltage represented by battery 54 is typically an external generator while the voltages of batteries 50 and 56 are conveniently provided by a DC to DC converter in the well-known manner wherein a transformer having a pair of output windings is utilized to provide the voltages of batteries 50 and 56; the same transformer is also used to supply filament current (not shown) for electron tubes 68 and 70.

Modulator 22 comprises a biasing network 57 of resistors 58, 60, 62, 64 and diode 66 which in combination with electron tube 68 provides a bias voltage between grid 40 and cathode 38 of x-ray tube 34, a pair of electrical elements having alternatively a state of conduction and a state of nonconduction herein represented by electron tubes 68 and 70 for supplying current to the biasing network 57, and a pulsing or keying circuit 71 comprising transistors 72 and 74 for supplying grid voltages to electron tubes 68 and 70 for inducing the states of conduction and nonconduction.

Transistor 72 of the keying circuit 71 comprises an emitter electrode 76 connecting with terminal V, which is the positive terminal of battery 54, a base electrode 78 and a collector electrode 80. Resistor 82 connects between signal source 26 and base electrode 78 to couple an input signal represented by pulse 28 from the signal source 26 to the transistor 72. Resistor 84 connects between base electrode 78 and terminal V, and in combination with resistor 82 provides voltage bias across the base emitter junction of transistor 72. Resistors 86, 88 and 90 and diode 92 are connected serially between the collector electrode and the negative terminal of battery 54 to provide a path for current through the transistor 72, the current entering transistor 72 at emitter electrode 76 and exiting via collector electrode 80. An alternative current path is provided by resistor 94 which couples the junction of resistor and diode 92 to terminal V: which is the negative terminal of battery 56. The current path depends on the signal applied to the base terminal 78 as follows. When the signal applied to the base terminal 78 is a negative pulse such as pulse 28, the resulting signal appearing at collector terminal 80 is a positive pulse such as pulse 96. Prior to the application of pulse 28 a sufficiently small current flows from terminal V, through transistor 72 and then through resistors 86, 88, 90 and 94 to terminal V during which time diode 92 is back biased and nonconducting. During the application of pulse 28 to base terminal 78 a relatively large current flows from V, through transistor 72 and resistors 86, 88, 90 and diode 92 which is now in a state of conduction due to the increased voltage developed cross resistor 94. Durnecting with the positive terminal of battery 56, a base terminal 100 connecting to the junction of resistors 90 and 94, and a collector terminal 102 which is coupled by a resistor 104 to terminal V Transistor 74 is normally in a state of conduction with current entering the transistor 74 at emitter electrode 98 and exiting via base electrode 100 and collector electrode 102 and thence passing respectively through resistors 94 and 104 to terminal V During the application of pulse 28 to transistor 72 the voltage drop across resistor 94 becomes sufficiently large to provide a reverse bias voltage across the base emitter junction of transistor 74 which terminates the current flow through transistor 74. Diode 92 constrains this reverse bias voltage to a value of less than approximately one volt and thereby protects the base emitter junction of transistor 74. During the duration of pulse 28 when the current in transistor 74 is terminated, the voltage across resistor 104 drops to zero thereby providing a negative output pulse, shown as pulse 106, from the keying circuit 71.

The biasing network 57 provides normally a negative voltage bias between the grid 40 and cathode 38 of x-ray tube 34 so that there is no electron beam current flowing in the x-ray tube 34. As is well known grid generally has the form of a cup which partially encloses the cathode 38 and serves to focus a beam of electrons emanating from the cathode 38 and flowing towards the anode 40. In x-ray tubes the anode 36 is frequently referred to as a target, and the cathode 38 is frequently formed from filaments heated by a low voltage filament supply which is connected to a negative terminal of a high voltage generator such as high voltage generator 30. An electron beam current flows through x-ray tube 34 when the grid-to-cathode voltage is reduced from the relatively large negative value to a value, for example, such as the slightly positive value provided by the forward voltage drop across diode 66.'The beam current ceases when the grid-tocathode voltage reverts to the negative value. These two values of grid-to-cathode bias voltage are provided with the aid of electron tubes 68 and 70 in the following manner.

The electron tube 68 is normally conducting and electron tube 70 is normally nonconducting. Electron tube 68 is coupled to the biasing network 57 via resistor 108 (having a value of typically 470 kilohms) which connects from the junction of resistors 60 and 62 to the anode 1 10 of electron tube 68. The grid 1 12 of electron tube 68 is connected to the junction of resistor 104 and the collector 102 of transistor 74, and the cathode 114 of electron tube 68 is connected to the negative terminal of battery 52. Resistors 58 and 60 are serially connected between terminal V which is the positive terminal of battery 50 and resistor 108 so that current flows from terminal V through resistors 58, 60 and 108 and thence through electron tube 68 and battery 52 to the negative terminal of battery 50. During this current flow the voltage at the junction of resistors 60 and 108 is negative with respect to the cathode 38 of x-ray tube 34 thereby providing the negative grid bias which inhibits the flow of electrons in the electron beam of x-ray tube 34. When a negative pulse such as pulse 106 from keying circuit 71 is applied to the grid 112 of electron tube 68 a negative voltage is developed between grid 112 and cathode 114 which terminates the current in electron tube 68. Thus, keying circuit 71 in combination with electron tube 68 function as a gate controlling current in electron tube 68.

When electron tube 658 is nonconducting, current flows from terminal V through resistors 58, 60, 62 and 64 and diode 66 to the negative terminal of battery 50. This flow of current provides a forward bias voltage across diode 66 and thus a forward bias voltage between grid 40 and cathode 38 so that an electron beam flows between the cathode 38 and the anode 36 of x-ray tube 34. It is noted that several diodes may be serially connected in lieu of diode 66 to reduce the inverse voltage rating required on an individual diode, a large inverse voltage rating being required because of the voltage supplied by battery 52. For example, five avalanche diodes of type 1N45ll have been utilized. Also, due to the large voltage supplied by battery 52, the resistors 58 and 64 are high voltage resistors. Resistor 64 is also of a relatively high resistance for example, 300 kilohms, to protect the circuitry of modulator 22 in the event of an arc in x-ray tube 34.

It is noted that keying circuit 71 utilizes direct coupling from the signal sources 26 to the grid 112 of vacuum tube 68. This direct coupling is attained by means of resistor 82 which couples the signal source 26 to the transistor 72, and then the series of resistors 86, 88, and 94 by which transistor 72 is coupled to transistor 74, and finally resistor 104 by which the output signal of transistor 74 is generated for electron tube 68. Thus, the keying circuit 71 is responsive to a pulse 28 having any desired width and to a step function, such as an input pulse of very long or indefinite duration. Similarly, the keying circuit 71 is responsive to a pulse 28 having a very short duration, the response being limited only by stray capacitances in the circuit.

In order to minimize the effect of stray capacitances and particularly the capacitance within the grid circuit of the x-ray tube 34, electron tube 70 which, as will be described below, is responsive substantially to a derivative of the pulse 96, is utilized to effect a rapid change in the voltage which is provided to the grid circuit of x-ray tube 34 by the biasing network 57. Electron tube 70 has an anode 116 connected to terminal V a cathode 118 connected to the junction of resistors 108 and 60, and a grid 120 connected to the output winding of transformer 122. Transformer 122 is a pulse transformer which, in response to the leading edge of pulse 96, generates a pulse of relatively short duration indicated by pulse 124. For example, transformer 122 has preferably a ferrite core 121 and electrostatic shield 123 between the input and output windings, and may be of small physical size measuring approximately 1% inch by 1%; inch by /8 inch. Transformer 122 has high voltage insulation to prevent voltage breakdown and leakage current flow from the secondary winding to the primary winding. It is noted that the use of transformer 122 eliminates the necessity of a second keying circuit such as is utilized with floating deck modulators of the prior art, thereby providing a substantial decrease in the size of the circuitry. In response to a pulse 124 applied to the grid 120, electron tube 70 conducts a current which flows from terminal V through electron tube 70 and then through resistor 62 and 64 and diode 66 to the negative terminal of battery 50. Since electron tube 68 is now nonconducting due to the presence of pulse 106, as described above, there is now no negative bias voltage between grid 120 and cathode 118 of electron tube 70 with the result that electron tube 70 continues to conduct current until the termination of pulse 106. The sudden impulse of current provided by electron tube 70 rapidly alters the grid-to-cathode voltage of x-ray tube 34 and thereby provides a sharply defined pulse of electrons in the electron beam of x-ray tube 34.

Diodes 126 and 128, respectively, in the input and the output windings of transformer 122 inhibit the formation of a pulse of opposite polarity to pulse 124 thereby insuring that the electron tube 70 retains its state of conduction. Diode 130 is connected from the collector terminal 80 to the emitter terminal 76 of transistor 72 to protect the transistor 72 from the voltages developed across the input winding of transformer 122. The polarity of the input winding of transformer 122 relative to the polarity of the output winding of transformer 122 is indicated in a conventional manner by means of the two little squares 132 shown adjacent each winding of the transformer 122. Capacitor 134 is connected in parallel with resistor 90 to provide a relatively low impedance for current passing through the input winding of transformer 122 during the occurrence of the leading edge of pulse 96 in order to provide a more sharply defined waveform of pulse 124 at the output winding of transformer 122. A zener diode 136 is connected in parallel with resistor 108 and is in a state of conduction in the reverse direction when electron tube 68 is in a state of conduction. Zener diode 136 functions to limit the voltage across resistor 108 and similarly the voltage appearing between the grid 120 and the cathode 1 18 of electron tube 70 during such time when electron tube 68 is conducting to prevent excessive negative grid-to-cathode voltage bias. Also, the zener diode 136 reduces the impedance of the grid-cathode circuit of electron tube 70 during the duration of pulse 124, the voltage magnitude of pulse 124 being greater than the zener voltage of zener diode 136, to effect increased conduction within electron tube 70. Electron tubes 68 and 70 are designed for high anode-to-cathode voltage and have low leakage current when biased OFF; thus when the x-ray tube 34 is being pulsed there is no more than a negligible voltage drop across resistor 108 due to such leakage current in electron tube 68, thereby insuring conduction within electron tube 70.

In operation, therefore, in response to an input signal in the form of pulse 28, keying circuit 71 provides an output pulse in the form of pulse 106 having a duration equal to the desired exposure time of an x-ray pulse from, x-ray tube 34. Pulse 106 may have a long or a short duration in accordance with the duration of pulse 28 since transistor 74 is directly coupled by resistors to transistor 72. A transformer 122 responsive to those frequencies in the spectrum of pulse 28 which are associated with the leading edge of pulse 28 provides an output responsive substantially to the derivative of pulse 28 (or equivalently pulse 96), this output consisting of a positive pulse 124 and a negative pulse which is of negligible value due to the limiting action of diodes 126 and 128. Electron tube 68 is normally conducting and electron tube 70 is normally nonconducting with the result that a negative voltage bias is applied to the x-ray tube 34 via biasing network 57 and electron tube 68. In response to pulses 106 and 124, electron tubes 68 and alter their states of conduction thereby altering the voltage bias to turn ON the electron beam in the x-ray tube 34.

Referring now to FIGS. 3 and 4 there is shown in FIG. 3 a block diagram of a system for modulating the beam of an electron beam tube such as a traveling wave tube and particularly an x-ray tube, and in FIG. 4 there is shown a housing 138 adapted to contain both an x-ray tube, not shown, and components of the modulation system of FIG. 3. The modulation system of FIG. 3 comprises the components of the modulation system 20 shown in FIG. 1, namely, signal source 26, modulator 22, load 24, fault detector and interlock 32 and a high voltage generator 30. The signal source 26 comprises an exposure control unit 140 located preferably at a remote panel accessible to an operator for selecting the width and repetition rate of the pulse 28 for control of the electron beam in the load 24. Load 24 is presumed to be an x-ray tube. The exposure control unit 140 generates typically a pulse 142 having a duration equal to that of pulse 28 which is transmitted by an electrical cable 144 to a junction box 146 affixed to the outside of housing 138. Pulse 142 is applied to an amplifier 148 which drives a light source 150 to produce a pulse of light having the same pulse duration as pulse 142. The light pulse is then passed through a light conduit 152 and received by a photo detector 154 which utilizes the optical energy to provide an electrical pulse which is amplified by amplifier 156 to provide the pulse 28. The use of a light conduit provides for electrical insulation between high voltage circuitry and circuits connecting with the exposure control unit 140 and, furthermore, eliminates the need of additional electrical conductors in high voltage cabling which provides electrical power for the modulator 22 and the x-ray tube 34. It is readily appreciated that many forms of radiation may be utilized to provide such insulation for example, infra-red light and visible light, or even acoustic radiation may be utilized with suitable transducers and sound conduits. Such radiation coupling also provides DC signal coupling.

As shown in FIG, 4 a light source in the form of a gallium arsenide diode 158 is utilized to transmit infra-red radiation having a wavelength of approximately 9,000 Angstroms through a light pipe 160 composed of a glass rod which is coated with another glass of higher refractive index to retain the radiation within the light pipe 160. Light pipe 160 passes through an aperture in the housing 138 and is directed towards a phototransistor l62which receives the infra-red radiation. Two printed circuit boards 164 and 165 are enclosed within a cylindrical metallic shield 166 and are spaced apart by insulating posts 167 and connected thereto by means of nylon screws 168. Printed circuit boards 164 and and 165 support phototransistor 162 as well as amplifier 156 and the circuitry of modulator 22 which are not shown in FIG. 4.

On the underside of printed circuit board 165 are a lead shield 169 and a plug 170 which together with the printed circuit boards 164 and 165 and the shield 166 form a module 171 which may be extracted from the housing 138 for servicing of the electrical circuitry on the printed circuit boards 164 and 165. The module 171 is enclosed within a rigid shell 172 of an insulating material such as plastic. The entire housing 138 is filled with oil, as is well known, for cooling the electrical components and for providing high voltage insulation.

In the base of shell 172 there is formed an electrical socket 173 which mates with plug 170. Socket 173 provides for electrical contact between the prongs 174 of plug 170 and the prongs 175 of an x-ray tube encased within the housing 138. Electrical circuitry within the module 171 is shielded from x-radiation by means of a lead shield 176 affixed tothe interior of housing 138, a lead shield 177 affixed to the shell 172 and the lead shield 169 of the module 171. Both lead shield 176 and 177 have apertures through which electrical connection is made from the module 171 to an x-ray tube. Openings, not shown in FIG. 4, may be provided in plug 170 and electrical socket 173 to permit circulation of the oil throughout the housing 138. The outer diameter of lead shield 177 is greater than the diameter of the aperture in lead shield 176 and, similarly, the diameter of the lead shield 169 is greater than the aperture in the lead shield 177 thereby providing for the interception of x-radiation. The module 171 is rigidly positioned within the shell 172 by means of an extension 178 of plug 170 which issecured to the shell 172 by means of nylon screws 179. The prongs 174 of plug 170 are connected to the circuitry of modulator 22 by electrical wires 180 which pass around the lead shield 169 and then enter the printed circuit board 165. I

Housing 138 closely resembles a typical x-ray tube housing and has a window 181 through which x-rays are transmitted to an object to be photographed as well as connectors 182 and 183 for the connection of high voltage cables. Electrical connection between connector 183 and socket 173 is made by electric wires such as wire I84 passing through an aperture in the shell 172. The modulator 22'has been enclosed within the housing 138 by extending it approximately 2% inches beyond the standard dimensions of such housing. Re-' ferring to FIG. 2 it is noted that the high voltage componentsare electron tubes 68 and 70, the biasing network 57 and the transformer 122. These components are mounted on printed circuit boards 165. The remaining components of the modulator 22 are mounted on printed circuit board 164. Since most of these components are of low voltage they are accordingly of small physical size and therefore can be mounted on the single printed circuit board 164. For service and maintenance of the module 171, end cap 185 is removed from the housing 138, the light pipe 160 and a second light pipe to be described hereinafter are pulled out of the way, and then the module 171 is slidaway from the lead shield 177 thereby disconnecting plug 170 from the electrical socket 173.

The fault detector and interlock 32 comprises amplifiers 186 and 188 having a common input terminal designated C in FIGS. 2 and 3 which is connected to the junction of resistors 60 and 62 in FIG. 2. Amplifier 186 has a second input terminal designated J which is connected to the junction of resistors 58 and 60. Amplifier 188 has a second input terminal designated K connected to the junction of resistors 62 and 64. Thus amplifier 186 is responsive to the voltage drop across resistor 60, and amplifier 188 is responsive to the voltage drop across resistor 62. Typical values for the resistors of the biasing network 57 are by way of example: Resistor 58 has a value .of 'megohms, resistor 60 has a value of 27 kilohms, resistor 62 has a value of 680 ohms, and resistor 64, as was mentioned earlier, has a value of 300 kilohms.

The relatively small resistance of resistor 62, as compared tothe resistance of resistor 64 has been selected to providea voltage drop of suitable magnitude for the input terminals of amplifier 188, and similarly the relatively small resistance of resistor 60 as compared to the resistance of resistor 58 has been selected to provide a voltage drop of magnitude suitable for the input terminals of amplifier 186. Thus, prior to the application of pulse 28 to the modulator 22, when current is flowing from terminal V through resistors 58, 60 and 108 and electron tube 68, the voltage applied across terminals J and C of amplifier 186 is a measure of the value of bias voltage from grid 40 to cathode 38 of x-ray tube 34. In a similar manner the voltage applied across terminals C and K of amplifier 188 indicates the value of reverse current flowing through diode 66. For example, as was mentioned above, diode 66 may be a series of five diodes of the avalanche type in which case there is an inverse current flowing through these diodes when the negative bias voltage is applied across the diodes. In the event that one of these diodes is damaged as by shorting by an arc in tube 34 excessive inverse current flows through these diodes with an attendant decrease in the value of the bias voltage. The excessive current results in an increase of voltage drop across resistor 62 and thereby a signal is presented to amplifier 188 indicative of the diode damage.

The output voltages from amplifier 186 and 188 are applied to a logic circuit 190 which deenergizes a light source 192 in the event of a voltage indication from either amplifier 186 or 188. In particular, it is noted that the light from light source 192 is extinguished whenever the voltage drop across resistor 60 falls below a preset value as when electron tube is in a stage of conduction. Thus the light is extinguished while the x-ray tube 34 is being pushed. In addition, the light provided by light source 192 is extinguished whenever a fault arises in the circuitry controlling the bias voltage for x-ray tube 34 thereby indicating a malfunction. The extinguishing of the light from light source 192 can result in a turning OFF of the high voltage generator 30 in the following manner; Light source 192 is a gallium arsenide diode such as that of light source 150, and its light is transmitted through a light conduit 194 such as the light pipe 196 shown in FIG. 4. Light conduit 194 conducts the light to a photo detector 198, preferably a phototransistor such as that employed in photo detector 154. Photo detector 198 utilizes the light energy to provide an electrical signal which is then amplified in amplifier 200 and transmitted to logic circuit 201 which may be conveniently mounted in the remote control panel with the exposure control unit 140.

The purpose of the logic circuit 201 is to determine whether the absence of light from light source 192 indicates a fault or indicates that the x-ray tube is being pulsed. Accordingly, when pulse 142 is applied to the logic circuit 201, the absence of the light is indicative of favorable operation of the modulator 22 while the presence of the light is indicative of a fault. During the absence of a pulse 142, the presence of the light indicates favorable operation while the absence of the light indicates a fault. In response to a determination of fault, the logic circuit 201 deenergizes relay 202 inthe high voltage generator 30. Relay 202 is typically placed in the transformer circuit providng the high voltage so that deenergization of relay 202 results in a disconnection of the high voltage. Photo detector 198 and amplifier 200 are conveniently located within the junction box 146. Electricalconnection from amplifier 200 to logic circuit 201 is provided by cable 144. In FIG. 3 a

dashed line 204 has been provided to show those electrical components which are located within the housing 138 and therefore exposed to high voltage, and those components which are located outside the housing 138 and therefore insulated from the high voltage by means of the light conduits 152 and 194.

It is understood that the above described embodiment of the invention is illustrative only and that modification thereof will occur to those skilled in the art. Accordingly, it is desired that this invention is not to be limited to the embodiment disclosed herein but is to be limited only as defined by the appended claims.

What is claimed is:

1. In combination:

a first element having alternatively a state of conduction and a state of nonconduction, said first element having a control terminal and being responsive to a control signal applied to said control terminal for providing said states of conduction and non-conduction;

' a pulse circuit coupled to said first element and responsive to an input signal from a signal source to generate said control signal, said pulsing circuit being directly coupled between said signal source and said first element;

an electron beam tube;

a second element having alternatively a state of conduction and a state of nonconduction;

a biasing network interconnecting said electron beam tube and said second element, said biasing network including a first resistor circuit coupled between said second element and said electron beam tube, said biasing network including a second resistor circuit coupled between a pair of terminals of said second element and coupled to said first resistor circuit to permit a measurement of bias voltage applied to said electron beam tube;

said first element being coupled between said pulsing circuit and said second element, said first element and said second element being serially connected between a source of power to provide for a modulation of the electron beam in said electron beam tube in response to the states of conduction and nonconduction of said first element and of said second element; and

means coupled between said pulsing circuit and said second element for providing a signal representing the derivative of the leading edge of said input signal, said derivative signal being applied to a control terminal of said second element to impart to said modulation of said electron beam a rapid change in the intensity of said electron beam.

2. A modulator circuit for varying the voltage of a first terminal of an electron beam tube relative to a second terminal of said electron beam tube which is energized from a high voltage supply, said modulator circuit comprising:

a first electron tube having a cathode, an anode, and at least one grid, said cathode being in circuit with said first terminal;

a second electron tube having a cathode, an anode,

and at least one grid;

a keying circuit responsive to an input signal and directly coupled between a source of said input signal and a grid of said first electron tube, a grid of said second electron tube being coupled to said keying circuit; said keying circuit comprising a first transistor and a second transistor coupled thereto by a resistor circuit;

said first electron tube and said second electron tube being serially connected between said high voltage supply, said modulator circuit further comprising a biasing circuit interconnecting the anode of said first electron tube and the cathode of said second electron tube with said second terminal of said electron beam tube to effect a rapid change in voltage at said second terminal at the beginning of said input signal; and

means coupled to said resistor circuit of said keying circuit for providing a signal representing the derivative of the leading edge of said input signal, said derivative signal being coupled to said second electron tube for further imparting a rapid change in voltage at said second terminal at the beginning of said inputsignal.

3. A modulator circuit comprising:

a transistor circuit comprising a first transistor and a second transistor, an output terminal of said first transistor being resistively coupled to an input terminal of said second transistor, an input terminal of said first transistor being coupled to a source of input signal;

a first electron tube and a second electron tube being serially connected between a source of power;

means coupled to said first transistor for forming a signal representing the derivative of the leading edge of an input signal from said source, said derivative means being coupled to said second electron tube for applying said derivative signal to a control terminal of said second electron tube, said second transistor being coupled to a control terminal of said first electron tube; and

a biasing network interconnecting a cathode of said second electron tube and an anode of said first electron tube, said biasing network having a terminal for coupling to a beam tube load which is energized by said source of power, said biasing network comprising a plurality of resistors coupled to an anode of said second electron tube and having terminals for permitting a measurement of bias voltage applied by said biasing circuit to said beam tube load, said derivative means cooperating with said serial interconnection of said first electron tube and said second electron tube to provide a rapid change in the voltage applied by said biasing circuit to said beam tube load. 

1. In combination: a first element having alternatively a state of conduction and a state of nonconduction, said first element having a control terminal and being responsive to a control signal applied to said control terminal for providing said states of conduction and non-conduction; a pulse circuit coupled to said first element and responsive to an input signal from a signal source to generate said control signal, said pulsing circuit being directly coupled between said signal source and said first element; an electron beam tube; a second element having alternatively a state of conduction and a state of nonconduction; a biasing network interconnecting said electron beam tube and said second element, said biasing network including a first resistor circuit coupled between said second element and said electron beam tube, said biasing network including a second resistor circuit coupled between a pair of terminals of said second element and coupled to said first resistor circuit to permit a measurement of bias voltage applied to said electron beam tube; said first element being coupled between said pulsing circuit and said second element, said first element and said second element being serially connected between a source of power to provide for a modulation of the electron beam in said electron beam tube in response to the states of conduction and nonconduction of said first element and of said second element; and means coupled between said pulsing circuit and said second element for providing a signal representing the derivative of the leading edge of said input signal, said derivative signal being applied to a control terminal of said second element to impart to said modulation of said electron beam a rapid change in the intensity of said electron beam.
 2. A modulator circuit for varying the voltage of a first terminal of an electron beam tube relative to a second terminal of said electron beam tube which is energized from a high voltage supply, said modulator circuit comprising: a first electron tube having a cathode, an anode, and at least one grid, said cathode being in circuit with said first terminal; a second electron tube having a cathode, an anode, and at least one grid; a keying circuit responsive to an input signal and directly coupled between a source of said input signal and a grid of said first electron tube, a grid of said second electron tube being coupled to said keying circuit, said keying circuit comprising a first transistor and a second transistor coupled thereto by a resistor circuit; said first electron tube and said second electron tube being serially connected between said high voltage supply, said modulator circuit further comprising a biasing circuit interconnecting the anode of said first electron tube and the cathode of said second electron tube with said second terminal of said electron beam tube to effect a rapid change in voltage at said second terminal at the beginning of said input signal; and means coupled to said resistor circuit of said keying circuit for providing a signal representing the derivative of the leading edge of said input signal, said derivative signal being coupled to said second electron tube for further imparting a rapid change in voltage at said second terminal at the beginning of said input signal.
 3. A modulator circuit comprising: a transistor circuit comprising a first transistor and a second transistor, an output terminal of said first transistor being resistively coupled to an input terminal of said second transistor, an input terminal of said first transistor being coupled to a source of input signal; a first electron tube and a second electron tube being serially connected between a source of power; means coupled to said first transistor for forming a signal representing the derivative of the leading edge of an input signal from said source, said derivative means being coupled to said second electron tube for applying said derivative signal to a control terminal of said second electron tube, said second transistor being coupled to a control terminal of said first electron tube; and a biasing network interconnecting a cathode of said second electron tube and an anode of said first electron tube, said biasing network having a terminal for coupling to a beam tube load which is energized by said source of power, said biasing network comprising a plurality of resistors coupled to an anode of said second electron tube and having terminals for permitting a measurement of bias voltage applied by said biasing circuit to said beam tube load, said derivative means cooperating with said serial interconnection of said first electron tube and said second electron tube to provide a rapid change in the voltage applied by said biasing circuit to said beam tube load. 